Preparation of printed circuits



Aug. 12, 1969 OUVER 3,460,448

PREPARATION OF PRINTED CIRCUITS Filed Sept. 7, 1966 3 Sheets-Sheet 1Auqm ys Aug. 12, 1969 3,460,448

PREPARATION OF PRINTED CIRCUITS Filed Sept. 7, 1966 3 Sheets-Sheet 2Attorneys Aug. 12, 1969 C. V. OLIVER PREPARATION OF PRINTED CIRCUITSFiled Sept. 7 1966 3 Sheets-Sheet 5 MAMA X y SYMBOL 2 3f, 0 50090 com/1wTABZE swam c0 0k0 SVMBUL 85156750 M5050 r/mve & ammo:

TR/P m 5 R5405? FZASH uv/r l n can (gr 914 WM Attorneys United StatesPatent 3,460,448 PREPARATION OF PRINTED CIRCUITS Cyril Victor Oliver,Stevenage, England, assignor to British Aircraft Corporation Limited,London, England, a British company Filed Sept. 7, 1966, Ser. No. 577,691Claims priority, application Great Britain, Sept. 13, 1965, 38,950/65Int. Cl. Hk 3/00; G03b 27/32 US. Cl. 951 4 Claims ABSTRACT OF THEDISCLOSURE A method and apparatus is disclosed for preparing masterprints from circuit diagrams in which data corresponding to the positionand character of each element of the tracks is first recorded on arecord strip. Silhouettes of the track elements are then arranged in acarrier in an optical system so that a light source passing therethroughimpinges on a light sensitive film or plate. The record strip is thenadvanced step by step and the data for each track element is sensed togenerate corresponding electrical signals. Silhouettes are thensuccessively selected in accordance with such signals and exposed ontothe light sensitive film or plate. Between exposures, the optical systemand light sensitive film or plate are moved relatively in one or both oftwo mutually perpendicular directions in accordance with the electricalsignals to position the film or plate for the next exposure. The masterprint is then obtained by developing the light sensitive film or plate.

In the manufacture of printed circuits the continuing reduction in sizeof electronic components is making the process of converting a designersoriginal circuit diagram to a finely drawn accurate master printincreasingly difficult, and therefore more time consuming. In one knownsystem the original circuit sketched by the designer is reproduced by adraughtsman in an enlarged form with tracks, bends and pads drawn towithin very fine tolerances. A condensed image of the draughtsmansreproduction is then photographed and the film developed to produce amaster print suitable for use in applying a photo-resist to a coppercoated printed circuit board.

According to the present invention, the master prints are obtained byrecording data corresponding to the position and character of eachelement of the tracks in the original circuit diagram on a record strip,arranging silhouettes of required track elements in a carrier in anoptical system and arranging a light sensitive film or plate in theexposure plane of the optical system, advancing the record strip step bystep and sensing the data for each track element in turn to generatecorresponding electrical signals, selecting silhouettes one afteranother in accordance with the electric signals and exposing eachselected silhouette onto the film or plate, and between exposuresrelatively moving the optical system and the film or plate in one orboth of two mutually perpendicular directions in accordance with theelectric signals to position the film or plate for the next exposure.Thus a reproduction of the original circuit is built up step by step onthe film or plate, and the required master print is obtained bydeveloping the film or plate.

Thus the invention enables the direct transference of an original sketchto a fixed master print without the need for the intermediate step ofpreparing a finely drawn copy. It also has the advantage that a tape ofa particular circuit can be easily stored and printed again or used in anew sequence where it forms part of a larger circuit, while the use ofsilhouettes enables a high degree of accuracy to be achieved.

The programmed data is normally fed to a control circuit which selectsthe required silhouette and permits an exposure flash only after therelative positioning of the film or plate and the optical system hastaken place, such that a silhouette is exposed in its correct position.

Individual silhouettes can be placed in the carrier as required butpreferably the carrier holds a complete range. In the latter case thesilhouettes may be individually illuminated from separate light sourcesor alternatively a single light source is employed which distributesillumination evenly over the whole range and an individual silhouette isexposed by masking the light passing through the remaining silhouettes.The silhouettes are conveniently arranged in a circle around thecarrier. A glass prism unit is then arranged to rotate beneath thecarrier and light is reflected through the unit such that it emergesparallel to its incident path and along the axis of rotation towards thefilm or plate. Thus light from any selected silhouette always emergesalong the same path, and the co-ordinate table need therefore only bemoved with respect to this path. At the same time this provides a methodof selecting the required silhouette when a single light source isemployed by timing the exposure flash to coincide with the prism beinglocated under the required silhouette. Alternatively with eachsilhouette individually illuminated, the selection simply comprisesswitching on the required lamp.

One example of the invention will now be described with reference to theaccompanying drawings in which:

FIGURE 1 is a schematic representation of a photocomposing apparatus;

FIGURE 2 is a perspective view of the co-ordinate table used in theapparatus of FIGURE 1;

FIGURE 3 is a sectional view through a portion of the apparatus ofFIGURE 1;

FIGURE 4 is a block circuit diagram of a control circuit for use withthe apparatus of FIGURE 1; and

FIGURE 5 is a schematic representation of a circuit composed with theapparatus of FIGURE 1.

Referring to FIGURES 1 and 2, a film 1 which is to be exposed is placedon a co-ordinate table 2. The optical system commences with a lamp (notshown) located at the centre of an annular prism 3 which evenlydistributes light from the lamp around the periphery of a plate 4 onwhich the prism rests. The light is reflected downwards through circularholes in the plate, each of which holds a condenser lens, and falls on acarrier plate 5 having corresponding holes in which are mountedsilhouettes of the track elements. Beneath this plate is a further plate6 containing collimation lenses and the light paths between thecondensing lenses and the collimating lenses are enclosed by tubularmasks 7 to reduce the effect of stray light. The three plates are' fixedrelative to one another in a frame 8.

The remainder of the optical system comprises a prism unit 9 and anobjective lens 10 and is shown in more detail in FIGURE 3. The prismrotates about the axis A-A and is shown passing under a collimation lens11.

The prism is held in an assembly including an upper plate 12 and a lowerplate 13 which rotate with respect to the plate 14 containing theobjective lens 10. The upper plate includes a hole 15 permitting lightincident from the collimation lens to pass through the prism unit. Theprism unit is out such that the light incident from the collimation lensfalls on the face 18 at an angle of 45 and is internally reflectedthrough 90 along the longitudinal axis of the prism. It then strikes thesecond face 19 at a point where this face intercepts the axis AA, isagain internally reflected through 90 and emerges downwards along theaxis AA. The emergent rays pass through a hole 16 in the lower plate andthrough the objective lens onto the film 1. Since the emergent opticalaxis of the prism unit lies on the axis AA an exposure flash through aselected silhouette at any point in the revolution of the prism unitresults in a light beam along this axis.

The co-ordinate table 2 is shown in more detail in FIGURE 2. The film islocated in a holder 20 which is mounted on a slide forming part of ablock 22 and can be moved along the slide in small steps in the Ydirection by the motor 21. The block 22 is in turn mounted on a slidewhich is perpendicular to the first slide and which forms part of afixed block 24. The block 22 can be moved in similar steps in the Xdirection by the motor 23. The figure shows a straightforward motordrive for simplicity but in practice a Geneva type drive mechanism isused.

To ensure that a correct reproduction of the original circuit is builtup step by step on the film, the exposure of the selected silhouettesmust be alternated with the positioning of the coordinate table. Withthe optical system described in this example all silhouettes areilluminated simultaneously during an exposure flash. The process ofselection therefore comprises timing the exposure flash to occur whenthe rotating prism lies beneath the collimation lens corresponding tothe required silhouette. After an exposure the co-ordinate table must bemoved to a new position before the next exposure occurs.

The operation of the whole process is controlled using the circuit shownin FIGURE 4. The original circuit is converted into data recorded on atape, the data including information concerning the type of trackelements required and the position they should occupy in an X, Yco-ordinate system.

The tape reader 30 applies corresponding electric signals for eachelement in turn to a memory 29 which separately stores the X and Ypositional data, the track element required and the step and repeatcommand. The positional data is fed as separate X, Y co-ordinates tocoordinate table servo mechanisms 31. The track elements are accordedsymbol numbers and the corresponding signals are fed to the symbolselector 32. The timing and control circuit 33 receives a confirmatorysignal from the co-ordinate table to indicate that the requiredco-ordinate position has been achieved. The symbol selector circuitreceives data representing at any moment the instantaneous angularposition of the prism unit. When the prism unit is in a positioncorresponding to the location of the required silhouette a furtherconfirmatory signal is sent from the symbol selector to the controlcircuit. Once the film holder has been positioned, the next signal fromthe symbol selector will cause the control circuit'to generate atriggering signal for a flash unit 34 to cause exposure of the selectedsilhouette on to the selected film position. A confirmatory flashmonitor signal indicates to the control circuit that the exposure hasbeen made.

The step-and-repeat facility will now be described. Consider the tapereader to have read a command containing the co-ordinates X Y and thesymbol S with the result that the symbol S has been printed at X Y Thenext command may have co-ordinates X Y and a symbol S The signal to thetable servos depends on the difference between X and X and between Y andY If a difference exists the table will move one step in either or bothdirections and the symbol S will be printed in the new position. X and Yare then compared with the new co-ordinates and if a difference stillexists the step and repeat command then repeats the process. S continuesto be printed each time the table moves one step until the differences XX and Y -Y are both reduced to zero, when the control circuit steps thetape reader to advance the tape to read the next line of data.

Thus the circuit is built up as shown in FIGURE 5. The elements 35making up the tracks correspond to the individual silhouettes. Since theco-ordinate table is moved in fixed steps the elements will be of equallength, but overlapping sutficiently to produce a continuous track onthe film.

In an alternative system, the silhouette carrier can be rotated and theprism kept stationary. Only a single condensing lens and a singlecollimating lens are then required.

I claim:

1. A method of preparing a master print from an original circuit diagramcomprising:

recording data corresponding to the position and character of eachelement of a track in said original diagram on a record strip;

advancing said record strip step by step to generate electric signalscorresponding to said data;

mounting silhouettes of required track elements in a circular arrayaround a carrier in an optical system; arranging a light sensitivesurface in the exposure plane of said optical system;

rotating a prism about an axis passing through the center of saidcircular array such that said prism passes in turn beneath eachsilhouette and each silhouette is thereby presented to a fixed point insaid exposure plane;

timing an exposure flash in accordance with said electric signals tocoincide with said prism lying beneath the silhouette of a track elementrepresented by said electric signals;

and between exposures relatively moving said optical system and saidsurface in at least one of two mut-ually perpendicular directions toposition said surface for the next exposure in accordance with saidelectrical signals, whereby a reproduction of the original circuit isbuilt up step by step on said surface;

and developing said surface to obtain the required master print.

2. Apparatus for preparing master prints from an original circuitdiagram comprising:

a carrier for holding a circular array of silhouttes of required trackelements; a light source for illuminating said array of silhouettes; aholder for a light sensitive surface; an optical system for exposingsaid silhouettes on to said light sensitive surface, said optical systemincluding a prism unit having two reflecting faces rotatable about anaxis passing through the center of said circular array and extendingoutwards so that light passing through a selected silhouette locatedabove said prism unit is internally reflected at a first of said facesand transmitted radially along the length of said prism unit to bereflected again at the second of said faces and to emerge along saidaxis;

driving means for causing relative movement between said optical systemand said surface in at least one of two mutually perpendiculardirections;

position control means for actuating said driving means responsive toelectric signals representing the required position on said surface of atrack element to be exposed; further driving means for continuouslyrotating said prism, and means responsive to electric signalsrepresenting a required track element for timing an exposure flash tocoincide with said prism lying beneath the silhouette of said requiredtrack element;

whereby a reproduction of the original circuit is built up step by stepon said surface from which the required master print is obtained.

3,460,448 5 6 3. Apparatus according to claim 2, including a 00-substantially even distribution with respect to said ordinate table towhich said holder is fixed and in which silhouettes. said driving meansincludes servo mechanisms for moving said table before exposure isinitiated, in accordance References Cited with said positionrepresenting sig nals. 5 UNITED STATES PATENTS 4. Apparatus according toclaim 2, 1n WhlCh said llght source comprises a lamp, an annular prismsurrounding 3'247761 4/1966 Herreman et said lamp, the outer peripheryof said prism extending around said circular array such that light fromsaid lamp NORTON ANSHER Pnmary Examiner is transmitted through saidprism on to said carrier in a 1 W. A. SIVERTSON, Assistant Examiner

